The usual procedure for manufacture of flexible polyurethane foam in block form involves metering several liquid chemicals in steady, continuous streams to a mixing head where they are intimately mixed together and deposited onto a film-covered conveyor which moves the chemicals forward. The chemical reactions produce polyurethane polymer and carbon dioxide gas which form, continuously, a block of polyurethane foam having a density which is usually in the range of 0.75 to 4.5 lbs/cu. ft. The reaction is exothermic. Chemicals exiting the mixing head at 68.degree. F. can reach a temperature of 330.degree. F., and sometimes more than 350.degree. F., at the time the block is fully formed, depending on the formulation used.
Conventionally, the essential chemicals used to make a polyurethane foam are polyol, toluene diisocyanate (TDI), water, a silicone surfactant, an amine catalyst, and stannous octoate catalyst. Other chemicals such as auxiliary blowing agents, flame retardants and inorganic or organic fillers may be incorporated optionally to meet specific requirements, but are not essential to the basic process.
The TDI reacts with the polyol to form polyurethane. The TDI also reacts with the water to form carbon dioxide gas, which serves as a blowing or foaming agent. The amine catalyst promotes the reaction between the TDI and water which forms carbon dioxide gas. The stannous octoate catalyst promotes the reaction between the TDI and the polyol to form the polyurethane polymer. The silicone surfactant serves to support the homogenization of the chemicals and to regulate the cell structure of the flexible polyurethane foam.
One method of manufacturing polyurethane foam block is where the mixed chemicals are deposited directly from the mixing head onto the fill covered conveyor. In this case the chemicals spread out, essentially unreacted, as a thin film. Reaction takes place as the chemicals are moved forward on the conveyor. The heat of the reaction is slow to develop and has to be accelerated by using moderate to high catalyst levels. This process, wherein the chemicals exiting the mixing head are immediately deposited onto the film covered conveyor as essentially unreacted chemicals, was prevalent until the late 1960's.
A well known and widely used improvement of this conventional procedure employs the Maxfoam.RTM./Varimax system. The Maxfoam.RTM. system was introduced in the late 1960's. With this system, the essentially unreacted chemicals from the mixing head are directed into the bottom of an open top trough, generally of 60-120 liters capacity. The chemicals start to react in the trough resulting in a froth which overflows the trough onto film-covered fall plates leading downwardly to a conveyor where further expansion and polymerization occurs. This system is the preferred method for producing flexible, polyether-type polyurethane foam, enabling higher blocks of foam to be produced at lower chemical throughputs and slower conveyor speeds. The Maxfoam.RTM./Varimax process simplifies operation and has important economic advantages.
One of the advantages of using a system employing a trough is that chemicals that enter a trough retain the heat of reaction much better, via a mass effect, and lower catalyst levels can be used compared to the conventional methods of depositing the chemicals directly onto the film covered conveyor. It can be appreciated that by varying the volume/capacity of the trough, that the deployment time or residence time in the trough can be varied. As an example, if the volume of the trough is doubled from 60 liters to 120 liters capacity, the froth exiting the trough will be thicker and at a more advanced stage in the overall blowing/polymerization process(es). As such, the trough can be considered to be a "mechanical" catalyst. For a given chemical throughput and catalyst levels, the degree of gas evolution and polymerization when the chemicals exit the trough (known as "spillover") will vary, dependent upon the volume of that trough and consequent chemical "dwell time" or residence time in that trough.
Present day polyurethane foam manufacturing plants can produce well in excess of fifty different grades of foam, from high to low densities and firm to soft grades. Reactivities of the chemicals used to manufacture different grades of foam vary widely and to accommodate this, different types and levels of catalysts and a range of trough sizes (volume/capacity) are used to optimize process conditions. It is desirable to avoid shutting down foam manufacture when changing grades, and wherever possible manufacturing runs of different grades which can be made with the same trough size are grouped together.
However, when changing to a grade which requires a different trough size, it may become necessary to stop the foam pouting operation, remove the trough, install a different size trough and then restart the operation, which is inconvenient and expensive.
U.S. Pat. No. 3,832,099 to Berg discloses a trough for forming polyurethane foams wherein partially expanded foam passes over a weir. To use a foam formation having a high rate of foam rise, the trough may be elevated vertically by a ram in order to cooperate with plates which are also vertically adjustable.
U.S. Pat. No. 4,032,275 to Schwab, et al. and U.S. Pat. No. 4,074,960 to Dockray, et at. also disclose troughs for forming polyurethane foam with height adjustment of the trough and conveyor. The volume of the trough may be varied by expansion of the trough in the direction transverse to conveyor movement to accommodate the production of varying widths of foam.
U.S. Pat. No. 4,093,109 to Schrader discloses a trough for forming polyurethane foams which has a curved lip portion. The trough also can be varied in volume by expansion of the trough in the direction transverse to conveyor movement.
Changing the height of a trough so as to accommodate various volumes of foaming ingredients creates problems in transfer of the foaming ingredients to a continuous belt conveyor. For example, changing the distance between the point of overflow and the belt conveyor may result in bubbling, or a rough or uneven top or bottom portion on the foam. The altered distance may require a change in the height or angle of the belt conveyor or a change in the angle of fall plates, an inclined conveyor, or a ramp between the trough and the belt. This angle change may result in an undesirable change in the shape or height of the foamed product.
Tilting of the trough may be used to adjust the distance between its overflow point and the belt conveyor. However, a conventional trough lip has an extended flat surface bent at an angle to the front wall of the trough to provide a downwardly directed flow surface. Tilting a trough with a conventional flat lip into different angular positions tends to create high shear forces and stresses upon the foaming ingredients. It may cause snagging of the foaming mixture and result in a non-uniform product, and excessive surface roughness or unevenness.
U.S. Pat. No. 4,530,807 to Vreenegoor discloses a trough for forming polyurethane foams which can be emptied by tilting at the end of a production run. Tilting to adjust trough volume for producing foam is not disclosed.
U.S. Pat. No. 4,559,003 to Poncet discloses a zone of retention for foam ingredients which may be formed above the nip of a rear cylinder and a front cylinder. In another embodiment, the zone may be formed by a rear wall which in part follows the coutour of a front cylinder. The volume of the zone can be adjusted continuously during polyurethane foam production by pivoting the rear cylinder or the rear wall about the axis of the front cylinder. It is disclosed that an advantage of the variable volume is to avoid the necessity of adjusting catalysis. The primary phase of expansion of the mixed chemicals and their further expansion are produced outside of the zone of retention on the horizontal surface of a plate and conveyor. However, a sheet continuously passes through the zone, and the front cylinder continuously rotates thereby requiring tight tolerances and subjecting the assembly to leakage and continual wear.
In the present invention, improved foam pouring flexibility is obtained by providing a variable volume trough wherein the volume/chemical retention time of the trough can be varied without requiring interruption of foam pouring for trough replacement. The foaming ingredients may be transferred from the trough to a substantially horizontal continuous conveyer belt at varied volume/retention times or residence times without requiring re-angling of any conveyer belts or re-angling of fall plates or ramps. The same or substantially the same inclination may be used for a plurality of dwell times without: a) creating undesirable bubbles, b) undercutting, c) adversely affecting foam surface uniformity and smoothness, and d) adversely affecting foam cell structure uniformity. The volume of the trough may be varied in accordance with the present invention to vary dwell lime by providing the trough with at least one moveable wall or by tilting the trough without the need for a sheet which continuously passes through the trough. In embodiments of the invention, the volume of the trough may be varied without changing the width of the apparatus and sheet of foam it produces. The trough of the present invention may be tilted to vary dwell time while permitting easy, snag-free positioning of the lip relative to a connecting fall plate or conveyor in different angular dispositions. The trough lip of the present invention achieves smooth overflow in different angular dispositions of the trough, and minimizes shear forces and stresses arising with extended lip structures.